In an electrical communications system, it is sometimes advantageous to transmit information signals (e.g., video, audio, data) over a pair of conductors (hereinafter “conductor pair” or “differential pair”) rather than over a single conductor. The conductors may comprise, for example, wires, contacts, wiring board traces, conductive vias, other electrically conductive elements and/or combinations thereof. The signals transmitted on each conductor of the differential pair have equal magnitudes, but opposite phases, and the information signal is embedded as the voltage difference between the signals carried on the two conductors. This transmission technique is generally referred to as “balanced” transmission. When a signal is transmitted over a conductor, electrical noise from external sources such as lightning, electronic equipment and devices, automobile spark plugs, radio stations, etc. may be picked up by the conductor, degrading the quality of the signal carried by the conductor. With balanced transmission techniques, each conductor in a differential pair often picks up approximately the same amount of noise from these external sources. Because approximately an equal amount of noise is added to the signals carried by both conductors of the differential pair, the information signal is typically not disturbed, as the information signal is extracted by taking the difference of the signals carried on the two conductors of the differential pair, and thus the noise signal may be substantially cancelled out by the subtraction process.
Many communications systems include a plurality of differential pairs. For example, the typical telephone line includes two differential pairs (i.e., a total of four conductors). Similarly, high speed communications systems that are used to connect computers and/or other processing devices to local area networks and/or to external networks such as the Internet typically include four differential pairs. In such systems, the conductors of the multiple differential pairs are usually bundled together within a cable and thus necessarily extend in the same direction for some distance. Unfortunately, when multiple differential pairs are bunched closely together, another type of noise referred to as “crosstalk” may arise.
“Crosstalk” refers to signal energy from a conductor of one differential pair that is picked up by a conductor of another differential pair in the communications system. “Crosstalk” includes both near-end crosstalk, or “NEXT”, which is the crosstalk measured at an input location corresponding to a source at the same location (i.e., crosstalk whose power travels in an opposite direction to that of an originating, disturbing signal in a different path), as well as far-end crosstalk, or “FEXT”, which is the crosstalk measured at the output location corresponding to a source at the input location (i.e., crosstalk whose power travels in the same direction as the disturbing signal in the different path). Both NEXT and FEXT are undesirable signals that interfere with the information signal.
A variety of techniques may be used to reduce crosstalk in communications systems such as, for example, tightly twisting the paired conductors (which are typically insulated copper wires) in a cable, whereby different pairs are twisted at different rates that are not harmonically related, so that each conductor in the cable picks up approximately equal amounts of signal energy from the two conductors of each of the other differential pairs included in the cable. If this condition can be maintained, then the crosstalk noise may be significantly reduced, as the conductors of each differential pair carry equal magnitude, but opposite phase signals such that the crosstalk added by the two conductors of a differential pair onto the other conductors in the cable tends to cancel out.
While such twisting of the conductors and/or various other known techniques may substantially reduce crosstalk in cables, most communications systems include both cables and communications connectors that interconnect the cables and/or connect the cables to computer hardware. Unfortunately, the communications connector configurations that were adopted years ago generally did not maintain the conductors of each differential pair a uniform distance from the conductors of the other differential pairs in the connector hardware. Moreover, in order to maintain backward compatibility with connector hardware that is already in place in existing homes and office buildings, the connector configurations have, for the most part, not been changed. As such, the conductors of each differential pair tend to induce unequal amounts of signal energy on each of the other conductors in current and pre-existing connectors. As a result, many current connector designs generally introduce some amount of NEXT and FEXT crosstalk.
FIG. 1 depicts an exemplary electrical communications system in which crosstalk is likely to occur. As shown in FIG. 1, a computer 101 is connected by a cable 102 to a modular jack 105 that is mounted in a wall plate 109. The cable 102 is a patch cord that includes a modular plug 103, 103′ at each end thereof and which contains a plurality (typically four) of differential pairs. Modular plug 103 inserts into a modular jack (not pictured in FIG. 1) provided in the back of the computer 101, and modular plug 103′ inserts into an opening 106 in the front side of the modular jack 105 so that the blades of the plug 103′ mate with respective contacts of the jack 105. In this manner, information signals may be communicated from the computer 101 to the modular jack 105. The modular jack 105 includes a connector assembly 107 at the back end thereof that receives and holds wires from a second cable 108 that are individually pressed into slots in the connector assembly 107 to make mechanical and electrical connection. The second cable 108 may connect the computer 101 to, for example, network equipment (not shown) thus enabling it to access a local area network (LAN) and/or the Internet (both not shown).
Pursuant to certain industry standards (e.g., the TLA/EIA-568-B.2-1 standard approved Jun. 20, 2002 by the Telecommunications Industry Association), the communication system of FIG. 1 may include a total of eight conductors 1-8 that comprise four differential pairs. By convention, the conductors of each differential pair are often referred to as a “tip” conductor and a “ring” conductor. The industry standards specify that, at the plug-jack mating point, the eight conductors are aligned in a row, with the four differential pairs specified as depicted in FIG. 2. As known to those of skill in the art, under the TIA/EIA 568, type B configuration, conductor 5 in FIG. 2 comprises the tip conductor of pair 1, conductor 4 comprises the ring conductor of pair 1, conductor 1 comprises the tip conductor of pair 2, conductor 2 comprises the ring conductor of pair 2, conductor 3 comprises the tip conductor of pair 3, conductor 6 comprises the ring conductor of pair 3, conductor 7 comprises the tip conductor of pair 4, and conductor 8 comprises the ring conductor of pair 4.
As shown in FIG. 2, in at least the connection region where the contacts (blades) of the modular plug 103′ (see FIG. 1) mate with the contacts of the modular jack 105, the conductors of the differential pairs are not equidistant from the conductors of the other differential pairs. By way of example, conductor 2 (i.e., the ring conductor of pair 2) is closer to conductor 3 (i.e., the tip conductor of pair 3) than is conductor 1 (i.e., the tip conductor of pair 2) to conductor 3. Consequently, differential capacitive and/or inductive coupling occurs between the conductors of pairs 2 and 3 that generate both NEXT and FEXT. Similar differential coupling occurs with respect to the other differential pairs in the modular plug 103′ and the modular jack 105.
U.S. Pat. No. 5,997,358 to Adriaenssens et al. (hereinafter “the '358 patent”) describes multi-stage schemes for compensating NEXT for a plug-jack combination. The entire contents of the '358 patent are hereby incorporated herein by reference as if set forth fully herein. The connectors described in the '358 patent can reduce the “offending” NEXT that may be induced from the conductors of a first differential pair onto the conductors of a second differential pair in, for example, the contact region where the blades of a modular plug mate with the contacts of a modular jack. Pursuant to the teachings of the '358 patent, a “compensating” crosstalk may be deliberately added, usually in the jack, that reduces or substantially cancels the offending crosstalk at the frequencies of interest. As discussed in the '358 patent, two or more stages of NEXT compensation may be provided, where the magnitude and phase of the compensating crosstalk signal induced by each stage, when combined with the compensating crosstalk signals from the other stages, provide a composite compensating crosstalk signal that substantially cancels the offending crosstalk signal over a frequency range of interest. The multi-stage (i.e., two or more) compensation schemes disclosed in the '358 patent can be more efficient at reducing the NEXT tlan schemes in which the compensation is added at a single stage, especially when the second and subsequent stages of compensation include a time delay that is selected and/or controlled to account for differences in phase between the offending and compensating crosstalk signals.
The applicable industry standards for connector crosstalk performance are generally specified in terms of NEXT and FEXT performance, and the required performance levels are usually specified for mated plug and jack combinations with the input terminals of the plug used as a reference plane. For example, communication channels using unshielded twisted pairs (UTP) of copper wire are often expected to at least meet industry “Category 6” standards which require at, for example, 100 MHz at least 54 dB NEXT loss and 43 dB FEXT loss with respect to any two signal paths through the mated connectors (i.e., the magnitude of the NEXT signal induced on any other differential pair when a first differential pair is excited must be at least 54 dB below the magnitude of the signal on the first differential pair at 100 MHz, and the magnitude of the FEXT signal induced on any other differential pair when a first differential pair is excited must be at least 43 dB below the magnitude of the signal on the first differential pair at 100 MHz). While the multi-stage compensation schemes according to embodiments of the '358 patent can allow connectors to meet the “Category 6” performance standards set forth in TIA/EIA 568B.2-1 standard, methods and systems for even further reducing the levels of NEXT and FEXT are desired.